Moose::Cookbook::Extending::Recipe3 - Providing an alternate base object class
version 2.0201
package MyApp::Base; use Moose; extends 'Moose::Object'; before 'new' => sub { warn "Making a new " . $_[0] }; no Moose; package MyApp::UseMyBase; use Moose (); use Moose::Exporter; Moose::Exporter->setup_import_methods( also => 'Moose' ); sub init_meta { shift; return Moose->init_meta( @_, base_class => 'MyApp::Base' ); }
A common extension is to provide an alternate base class. One way to do that is to make a MyApp::base and add extends 'MyApp::Base' to every class in your application. That's pretty tedious. Instead, you can create a Moose-alike module that sets the base object class to MyApp::Base for you.
MyApp::base
extends 'MyApp::Base'
MyApp::Base
Then, instead of writing use Moose you can write use MyApp::UseMyBase.
use Moose
use MyApp::UseMyBase
In this particular example, our base class issues some debugging output every time a new object is created, but you can think of some more interesting things to do with your own base class.
This uses the magic of Moose::Exporter. When we call Moose::Exporter->setup_import_methods( also => 'Moose' ) it builds import and unimport methods for you. The also => 'Moose' bit says that we want to export everything that Moose does.
Moose::Exporter->setup_import_methods( also => 'Moose' )
import
unimport
also => 'Moose'
The import method that gets created will call our init_meta method, passing it for_caller => $caller as its arguments. The $caller is set to the class that actually imported us in the first place.
init_meta
for_caller => $caller
$caller
See the Moose::Exporter docs for more details on its API.
To actually use our new base class, we simply use MyApp::UseMyBase instead of Moose. We get all the Moose sugar plus our new base class.
MyApp::UseMyBase
Moose
package Foo; use MyApp::UseMyBase; has 'size' => ( is => 'rw' ); no MyApp::UseMyBase;
This is an awful lot of magic for a simple base class. You will often want to combine a metaclass trait with a base class extension, and that's when this technique is useful.
Stevan Little <stevan@iinteractive.com>
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